I actually know this one! Yippee! If you've got builders, find the spot in the village where there's a plank and a rock and when a builder stands there you get a message like "the villager could try building here". If you need to encourage building skills, take any villager to an existing hut and, eventually, they'll start "fixing the hut" which increases their building skills. Just keep at this until you've got a few people working on building a new hut - I think the building site is to the west of the research table & the school about halfway between them. (I just created a new player to see where the original huts were - I'm so codependent it's really sad!).